循环数据量大，如何提高Graphics的绘图速度

Question

        public Bitmap DrawBitmap()
        {
            Bitmap bmp = new Bitmap(3720, 3720);
            Graphics sg = Graphics.FromImage(bmp);

            float st = 270;
            float ws = 0.5f;
            for (int i = 0; i < 730; i++)
            {
                st = st + ws;
                for (int j = 0; j < 1840; j++)
                {
                    int Ri = 1840 - 1 * j;
                    Rectangle rec = new(20 + j, 20 + j, Ri * 2, Ri * 2);
                    SolidBrush sb = GetRandomColor();
                    sg.FillPie(sb, rec, st, ws);
                }
            }

            sg.Dispose();
            return bmp;
        }

        private SolidBrush GetRandomColor()
        {
            long tick = DateTime.Now.Ticks;
            Random ran = new Random((int)(tick & 0xffffffffL) | (int)(tick >> 32));

            int R = ran.Next(255);
            int G = ran.Next(255);
            int B = ran.Next(255);
            B = (R + G > 400) ? R + G - 400 : B;//0 : 380 - R - G;
            B = (B > 255) ? 255 : B;
            return new SolidBrush(Color.FromArgb(R, G, B));
        }

请教大神，如何修改这段绘制Bitmap方法，提高绘图速度？不管用哪台电脑绘制下来基本上都要超过1分钟，有没有办法提高绘制效率呢。

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努力~